50 Fun Facts About Eyes

Roughly how wide is a typical adult human eyeball from front to back?

A typical adult eyeball measures about 24 mm from front to back and holds roughly 6.5 millilitres; at birth it is only about 70 percent of that length.

Which part of the eye provides most of its focusing power?

The clear dome is about 11.5 mm across and half a millimetre thick at the centre; the lens does the fine-tuning.

The human pupil's diameter ranges from about 2 mm up to roughly what in darkness?

In darkness the pupil opens to about 8 mm across, while in bright light it shrinks to roughly 2 mm; the muscular iris makes the adjustment.

How many muscles move each human eye?

Six extraocular muscles move each eye: four straight recti and two obliques, which together let it look in every direction and roll slightly.

Which photoreceptor cells in the retina work in dim light and see only in shades of grey?

The retina holds around 7 million cones and somewhere between 75 and 150 million rods.

Which part of the retina, packed with cone cells, delivers the sharpest central vision?

Fovea is the tiny central pit of the retina where cone cells are densest; it covers only the central two degrees of the visual field, which is why reading needs a direct gaze.

The retina is unusual among body tissues because it is technically part of what?

The central nervous system includes the retina, which grows out of the embryonic brain as an outgrowth of the diencephalon; its blood vessels are the only ones a doctor can inspect directly without surgery.

The natural blind spot in each eye is caused by what?

The optic nerve's exit point, the optic disc, has no photoreceptors, leaving a gap roughly 7.5 degrees high by 5.5 degrees wide that the brain fills in unnoticed.

The jelly-like substance filling the back of the eyeball is called what?

Vitreous humour is a clear gel, about 99 percent water with fine collagen fibres, that fills the large rear chamber and holds the eyeball's shape; the watery aqueous humour fills the front chambers.

Which thin membrane lines the eyelids' inside and covers the white of the eye?

Conjunctiva is the thin, clear mucous membrane lining the inner eyelids and covering the sclera; its inflammation, conjunctivitis, is the familiar pink eye.

A shortage of which vitamin is the classic cause of night blindness?

Vitamin A supplies retinal, the light-catching half of rhodopsin, the pigment behind dim-light vision; when it runs short the eyes struggle in the dark, a condition called nyctalopia.

What is the age-related loss of near focusing, common after 40, called?

Presbyopia develops as the eye's focusing element stiffens with age and can no longer adjust for close work, which is why reading glasses become common after about 40.

What gives blue eyes their colour, since the iris holds no blue pigment?

Light scattering in the stroma, the iris's front layer of collagen fibres, sends short blue wavelengths back out; it is the Tyndall effect, a cousin of the Rayleigh scattering that makes the sky blue.

Research suggests all blue-eyed people share a single ancestor whose HERC2 mutation arose roughly when?

Copenhagen researchers dated the change to 6,000 to 10,000 years ago; the HERC2 mutation dialled down the neighbouring OCA2 gene and with it the melanin that makes irises brown.

The yellowish pigment that gives amber eyes their colour is called what?

Lipochrome is the yellowish pigment deposited in the iris that gives amber eyes their golden, coppery colour; the shade is uncommon in humans but familiar in wolves and many domestic cats.

In a laboratory setting, adults blink on average about how many times per minute?

Resting adults averaged 17 blinks a minute in the classic study of spontaneous blinking, each lasting about 100 to 150 milliseconds; the rate falls to four or five while reading, which is why screens dry eyes out.

Newborn babies blink roughly how often?

Once or twice a minute is typical for a newborn, against well over a dozen blinks for a resting adult; the rate climbs steadily through childhood.

The tears you shed while chopping onions belong to which of the three types?

Reflex tears flood the eye in response to irritants such as onion vapour, dust or bright light; basal tears keep the surface wet all day and emotional tears are the third kind.

Which antibacterial enzyme in tears attacks the cell walls of certain bacteria?

Lysozyme breaks down the peptidoglycan cell walls of many bacteria; Alexander Fleming discovered it in 1922, well before his penicillin of 1928, after a drop from his nose fell onto a culture plate.

Red-green colour blindness affects roughly what share of men?

About 1 in 12 men have red-green colour deficiency, against only about 1 in 200 women; the responsible genes sit on the X chromosome and women carry a second copy.

Which English chemist, colour-blind himself, lends his name to a term for the condition?

John Dalton described his own colour blindness in 1798 and left instructions for his eyes to be examined after death; DNA from a preserved eyeball confirmed in 1995 that he had deuteranopia.

The dotted-circle plates used to screen for red-green colour deficiency are named after whom?

Ishihara plates were devised by the Japanese ophthalmologist Shinobu Ishihara in 1917; each shows a number or path in coloured dots that people with red-green deficiency read differently or cannot see.

In myopia, or short-sightedness, where does light from distant objects come to a focus?

In front of the retina is where a myopic eye focuses distant light, usually because the eyeball has grown slightly too long from front to back; outdoor time in childhood appears to protect against it.

A cataract is a clouding of which part of the eye?

The lens sits behind the iris and clouds with age; cataract surgery swaps the cloudy lens for a plastic implant, a technique Harold Ridley pioneered in London in 1949.

Eduard Zirm performed the first successful corneal transplant in 1905 in a town in what is now which country?

Czechia is home to Olomouc, the Moravian town, then in Austria-Hungary, where Zirm performed the graft on a labourer blinded by lime burns in December 1905.

The letter chart used to test eyesight was devised in 1862 by which Dutch ophthalmologist?

Herman Snellen published his chart in 1862 while working under Franciscus Donders in Utrecht; the classic version has eleven lines and uses only the letters C, D, E, F, L, O, P, T and Z.

A standard eye test is read from how far away?

That is where 20/20 comes from: seeing at 20 feet what a person with normal acuity sees at 20 feet. Reading only the big top letter is 20/200, legally blind in the US.

The first eyeglasses were made in about 1290 in which country?

Italy is the accepted birthplace, probably Pisa, where the earliest spectacles appeared between about 1286 and 1290; the story that a Florentine named Salvino D'Armate invented them has been debunked.

Which American statesman is credited with inventing bifocal spectacles?

Benjamin Franklin, who needed one pair of glasses for distance and another for reading, described his 'double spectacles' in a letter of 1785, with the distance half above and the reading half below.

In which city was Ibn al-Haytham, the 'father of modern optics', born?

Basra, in what is now southern Iraq, was his birthplace around 965; he later worked in Cairo, where his Book of Optics gave the first clear account of the camera obscura and influenced Kepler.

Which German ophthalmologist made the first successful contact lens, of blown glass, in 1888?

Adolf Fick, nephew of the physiologist of the same name, blew heavy glass shells that rested on the white of the eye and could be worn for only a few hours; Leonardo da Vinci had sketched the idea in 1508.

Which eye disease, usually caused by raised pressure inside the eyeball, slowly destroys peripheral vision?

Glaucoma damages the nerve fibres leaving the eye, typically because fluid pressure inside the eyeball is too high; it creeps in from the edges of vision, so it often goes unnoticed until damage is done.

The mantis shrimp's eyes contain how many distinct types of photoreceptor cell?

Their compound eyes hold 12 to 16 kinds of photoreceptor, far more than any human retina, yet experiments show they tell colours apart less finely than people; some species even detect circularly polarised light.

Which animal has the largest eyes ever known, up to about 40 cm across?

Colossal squid eyes are the largest known of any animal: a specimen examined in New Zealand in 2008 measured about 27 cm across even after partly collapsing, suggesting 30 cm or more in life.

Owls have to swivel their heads up to 270 degrees because their eyes are what?

Fixed in tubular sockets by bony rings, owl eyes cannot swivel at all, so the bird turns its head instead, helped by 14 neck vertebrae, twice the human count, and arteries with extra slack.

A chameleon can watch two different objects at once because it can do what?

'Move each eye independently' is exactly what a chameleon's turret-like eyes do, scanning nearly all round; when it hunts, both eyes swing forward for stereo vision before the tongue fires.

What shape are a goat's pupils?

Horizontal slits, almost rectangular, let a grazing goat take in a panoramic view along the ground while limiting dazzle from the sky; the pupils even rotate to stay level when the head tilts down.

Scallops' 200 tiny eyes focus light mainly with what?

A parabolic mirror, a concave tiling of square guanine crystals behind the retina, does the focusing, bouncing light forward onto the light-sensing cells the way a reflecting telescope does.

How many eyes does a box jellyfish have, despite having no brain?

Twenty-four eyes sit in four clusters, one on each sensory club called a rhopalium; two in each cluster are camera-type eyes with image-forming optics, the rest simple pits and slits.

The reflective layer that makes a cat's eyes shine in headlights, which humans lack, is called what?

Tapetum lucidum, Latin for 'bright tapestry', is a mirror-like layer behind the retina that bounces light back through it for a second pass, boosting night vision; humans have no such layer.

The red-eye effect in flash photographs is mainly caused by light reflecting off which blood-rich layer?

The choroid sits behind the retina and feeds it; the flash goes in and bounces straight back out red.

The combined field of view of both human eyes spans roughly how many degrees horizontally?

The full horizontal sweep is about 214 degrees, with each eye contributing a little over 100 degrees on its own side; roughly 120 degrees in the middle is seen by both eyes at once, giving depth perception.

Which retinal cells, discovered in the early 2000s, detect light and help set the body clock?

Photosensitive ganglion cells, identified in the early 2000s, carry the pigment melanopsin and signal overall brightness to the brain's master clock; they also drive the pupil's response to bright light.

What does the seventh extraocular muscle in each orbit do?

Lifting the upper eyelid is the task of the levator palpebrae superioris, whose name means just that; it runs from the back of the orbit to the lid, and a weak one gives a droopy eyelid.

In which direction does a single human eye see furthest, about 107 degrees from the fixation point?

Outward, toward the temple is where the field extends furthest because no facial feature stands in the way; the brow, the nose and the cheek all cut the upward, inward and downward fields shorter.

Which lobe of the brain initiates voluntary saccades, the rapid jumps both eyes make together?

The frontal lobe's frontal eye fields issue the command for voluntary saccades, which the brainstem then executes at several hundred degrees per second; smaller movements include drift, ocular tremor and microsaccades.

During accommodation for near objects, what shape does the lens relax into?

More convex is the shape it takes: the ciliary muscle contracts, slackening the suspensory fibres so it bulges rounder and bends light more strongly onto the retina.

What is the greyish-white ring of fatty deposits that can appear around the iris called?

Arcus senilis is a ring of cholesterol and other lipids deposited in the edge of the clear front of the eye; harmless in the elderly, it can signal high cholesterol in younger people.

Which two antioxidant pigments in the macula are linked to a lower risk of age-related macular degeneration?

Lutein and zeaxanthin, yellow carotenoids concentrated in the macula, absorb high-energy blue light and quench free radicals; a diet rich in leafy greens and eggs raises their levels in the eye.

After entering total darkness, roughly how long does it take for dark adaptation to be 80 percent complete?

Thirty minutes in darkness brings adaptation to about 80 percent, as the dim-light receptors rebuild their light-sensitive pigment rhodopsin; a burst of light bleaches it again and restarts the clock.